Method of making an article having a hard and ornamental coating

ABSTRACT

BEAUTIFUL AND GOLDEN COLORED ARTICLES ARE EFFECTIVELY MADE BY COATING OF TANTALUM CARBIDE ACCORDING TO THE INVENTION. THE COATING IS SO HARD THAT IT IS HARDLY SCRATCHED. THE METHOD COMPRISES PROVIDING A DISPERSION OF FINE POWDERS OF TANTALUM OR TANTALUM COMPOUND OR MIXED FINE POWDERS OF THE SAME WITH OTHER MATERIALS IN WATER AQUEOUS MEDIUM OR ORGANIC SOLVENT, APPLYING A DIRECT CURRENT VOLTAGE TO THE SURFACE OF A WORKPIECE TO BE COATED TO THUS CAUSE ELECTROPHORETIC DEPOSITION OF THE FINE POWDERS THEREON, HEATING THE COATED SURFACE AT HIGH TEMPERATURES IN VACUUM OR IN A REDUCING OR INERT ATMOSPHERE AND THEREBY FORMING A COATING LAYER CONSISTING MAINLY OF TANTALUM CARBIDE.

United States Patent 3,749,656 METHOD OF MAKING AN ARTICLE HAVING A HARD AND ORNAMENTA'L COATING Akio Hara and Shuji Yazu, Itami, .lapan, assignors to Sumitomo Electric Industries, Ltd., Osaka, Japan N0 Drawing. Filed Apr. 12, 1971, Ser. No. 133,382 Claims priority, application Japan, Apr. 20, 1970, 45/32,950 Int. Cl. B01k 5/02; C23b 13/00 US. Cl. 204-181 9 Claims ABSTRACT OF THE DISCLOSURE Beautiful and golden colored articles are effectively made by coating of tantalum carbide according to the invention. The coating is so hard that it is hardly scratched. The method comprises providing a dispersion of fine powders of tantalum or tantalum compound or mixed fine powders of the same with other materials in water, aqueous medium or organic solvent, applying a direct current voltage to the surface of a workpiece to be coated to thus cause electrophoretic deposition of the fine powders thereon, heating the coated surface at high temperatures in vacuum or in a reducing or inert atmosphere and thereby forming a coating layer consisting mainly of tantalum carbide.

BRIEF SUMMARY OF THE INVENTION This invention relates to articles having an ornamental coating which is golden colored, beautiful and hard so that it is hardly scratched and to a method of making such articles.

Gold platings have a beautiful and gold lustre, but are soft and readily scratched. Of late, WC-Co cemented carbides, called non-scratch, have been used for ornaments, for example, watch frames.

It is an object of the invention to provide parts or articles having a coating layer consisting mainly of tantalum carbide, based on the fact that tantalum carbide has a beautiful and gold color.

The above mentioned parts or articles of the invention are favourably compared with those the whole of which consists of an alloy consisting mainly of tantalum carbide in the following points:

(1) Very expensive tantalum carbide can be saved and, therefore, the production cost of equivalent articles is markedly reduced.

(2) Since TaC base alloys are very hard, articles consisting mainly of the same produced by the powder metallurgical or melting process are hardly machinable. For ornaments, in particular, there are various size and complex shaped articles, which are hardly machinable. This disadvantage can be overcome by selecting suitably a high machinability workpiece to be coated according to the invention, which is previously machined into a desirable shape.

It is another object of the invention to provide a method of forming a coating layer consisting mainly of tantalum carbide.

As a method of coating high melting point metals, graphites and steels with TaC base alloys, plasma melt spraying and electroplating are well known. In the method by melt spraying, the adhesive strength between a base member and coating layer and the strength of a coating layer itself are not sufficient. This method cannot be applied to base members of complex shape and, further, requires expensive apparatus. In the method by electroplating, on the other hand, carbide grains such as tantalum carbide are deposited simultaneously with electroplating of a binder metal such as nickel, resulting in that the quantity of the binder metal in a coating is so much that the color peculiar to tantalum carbide is lost.

3,749,656 Patented July 31, 1973 In the method utilizing electrophoretic deposition ac cording to the invention, a very dense, uniform and beautiful coating layer consisting mainly of tantalum carbide is economically obtained. Electrophoretic deposition has been put to practical use, for example, in coating a surface with an aqueous paint by electrodeposition using water as a dispersing medium or in electrodepositing fine powder of alumina on a heater of electronic tube. Furthermore, the utilization of electrophoretic deposition has been proposed as disclosed in Japanese patent publication No. 8951/1963, US. Pat. No. 3,171,192 and Journal of the Electrochemical Society vol. 109, No. 10, pp. 923-927.

DETAILED DESCRIPTION OF THE INVENTION We, the inventors, have succeeded in, by utilization of the electrophoretic deposition of this kind, that powders of tantalum or tantalum compounds, or their mixed powder with other materials are bonded to a base member such as high melting point materials, heat resistant alloys, cemented carbides, cermets, steels and pure iron and then subjected to a high temperature treatment at 1000 C. or higher, thereby promoting alloying reaction or solid solution making reaction of the bonded powder layer with the surface layer of the base member. That is to say, the feature of the method of forming a coating according to the invention consists in providing a dispersion of fine powders of tantalum or tantalum compounds or mixed fine powders of the same with other materials in water, aqueous medium or organic solvent, immersing a workpiece to be coated as one electrode in said dispersion, applying a direct current voltage of several ten to several hundred volts to said workpiece to thus cause electrophoretic deposition of the fine powders thereon, heating the coated surface at high temperatures in vacuum or in a reducing or inert atmosphere and thereby forming a coating layer consisting mainly of tantalum carbide.

The materials to be coated according to the invention include cemented carbides such as tungsten carbide-cobalt and tungsten carbide-cobalt with titanium carbide, tantalum carbide, molybdenum carbide, hafnium carbide, zirconium carbide, chromium carbide and/or vanadium carbide, high melting point metals such as tungsten and molybdenum, steels such as stainless steel and carbon steel, cermets and pure iron.

When tantalum or a tantalum compound besides tantalum carbide, for example, tantalum hydride or tantalum oxide is used, it is necessary to form tantalum carbide during the step of heating. In case where the material to be coated contains carbon, it is not necessary always to add fine powder of carbon, but, in general, a predetermined amount of fine carbon powder is preferably added to a starting powder followed by electrophoretic deposition.

The composition of the thus obtained coating layer comprises tantalum carbide as a main component and a transition metal such as iron group metal or copper as a binder metal. When this binder metal exceeds 50%, the hardness lowers to a large extent. Thus it is desired to hold the binder metal less than 50%. Moreover, carbides besides tantalum carbide, borides and nitrides may be contained therein as far as they do not deteriorate the beautiful color of tantalum carbide.

Preparation of the above mentioned dispersion of tantalum or tantalum compound may be carried out, as occasion demands, with addition of a small amount of a colloidal ion forming material, for example, alginates such as sodium, calcium, magnesium and ammonium alginates, nitrates such as aluminum nitrate, and sulfates to a dispersing medium, in particular, water or aqueous dispersing medium. In this case, a tantalum containing electrodeposited film excellent in adhesiveness can be obtained at a low voltage of several ten volts. An amount of about 0.05% based on the dispersing medium is sufficient. In the case of alginate, the alginate added to a dispersing medium is dissociated into alginate ion and metal ion, the alginate ion adhering to tantalum or tantalum compound, for example, tantalum hydride fine grains followed by electrophoresis to the surface of a workpiece to be coated as anode. With increase of the concentration of the dispersing system through aggregation of the power approaching the surface of a workpiece, the alginate tends to deposit. During the same time, it becomes a colloid and acts as a paste to strengthen the electrodepositing effect as well as an adhesive between the workpiece and fine powder.

In accordance with the method of coating fine powders of tantalum or tantalum compound utilizing electrophoresis, a film of uniform thickness can be formed on parts of complex shape or sharp angle and the thickness of the electrodeposited film can be freely controlled, for example, ranging from several microns to several hundred microns. The coating layer consisting of the electrodeposited fine powder sinters densely and adheres strongly to the base member during the next heating treatment. This is the most important effect of the invention, resulting in a coating layer more excellent in hardness and beauty as compared with the known method wherein a metallic layer is formed on a base member directly or by gaseous phase deposition or by electroplating and then heated at a high temperature to form a coating layer consisting of a solid solution carbide as disclosed in French Pat. 1,525,512.

In the case of using tantalum carbide fine powder, an alloying or solid solution making reaction with the surface layer of a workpiece to be coated takes place intimately, whilst, in the case of using tantalum or tantalum compound fine powder besides tantalum carbide, a carbonization reaction with previously mixed carbon, diffused carbon from a workpiece to be coated or carbon in atmosphere proceeds. When the base member is of cemented carbides, in particular, its binder metal for example, cobalt moves during heating like capilarity and, consequently, the coating layer containing the moved binder metal is obtained. Of course, a material to be the binder metal may previously be electrodeposited. When a workpiece to be coated and tantalum carbide form a low melting point liquid phase, the metal penetrates through tantalum carbide by capilarity, liquid phase sintering taking place, even if the powder to be electrodeposited contains no binder metal. In this case also, a metal capable of forming a low melting point liquid phase with tantalum carbide may previously be plated onto a workpiece.

The following examples are given in order to illustrate the invention in detail without limiting the same.

Example 1 Tantalum carbide powder of -200 mesh was ballmilled by wet process for 3 hours and dried. g. of the resulting powder was taken and dispersed in a mixed solution of 50 ml. of methylene chloride, 30 ml. of isopropyl alcohol and 20 ml. of nitromethane. 0.1 g. of prolamin was added thereto and stirred for 3 hours. A nickel plate was used as anode and an insert (12.7 mm. square, 4.8 mm. thickness) of K20 cemented carbides (WC-7% Co) was held by a narrow copper wire as cathode. 200 volts direct current at milliamperes was applied across the electrodes for 60 seconds to effect electrophoresis. The thus coated insert was then dried in the air and held at 1350 C. for 1 hour in a vacuum furnace mm. Hg).

The resulting coating layer was a uniform and completely dense layer of about 100 microns, which gave a beautiful golden color when lapped.

Example 2 A commercially sold tantalum hydride powder of 325 mesh was ball-milled by wet process for 15 hours to obtain a colloidal powder of tantalum hydride. 15 g. of the resulting powder was taken and dispersed in 300 ml. of water. Moreover, I g. of powdered sodium alginate was dissolved in 500 ml. of water to prepare a solution and 10 ml. of this solution was added to the dispersion with agitation. A copper plate was used as cathode and an insert (12.7 mm. square, 4.8 mm. thickness) of K20 cemented carbides (WC-7% Co) was held by a narrow copper wire as anode. 10 volts direct current at 0.5 milliampere was applied across the electrodes for 60 seconds to effect electrophoresis. The thus coated insert was then dried in the air and held at 1350 C. for 1 hour in a vacuum furnace (l0- mm. Hg).

The resulting coating layer was a uniform layer of about 60 microns, which gave a beautiful golden color when lapped. X-ray diffraction showed nothing on the surface of the insert but the diffraction figure of tantalum carbide.

Example 3 A commercially sold tantalum hydride powder of 325 mesh was mixed with 5% of Cobalt powder and 6.2% of carbon powder and ball-milled by wet process for 24 hours to obtain a colloidal mixed powder. A workpiece of molybdenum was used as anode in place of K20 cemented carbides and subjected to electrodeposition under the similar condition to that of Example 2. The thus coated workpiece was then dried in the air and held at 1350 C. for 30 minutes in a hydrogen furnace.

The resulting coating layer was a uniform layer of about 50 microns in thickness, which gave a beautiful gold color when lapped.

Example 4 Tantalum carbide powder of -200 mesh and 10% of electrolytic copper powder of mesh were ball-milled by wet process for 24 hours. The colloidal mixed powder was subjected to electrodeposition to a workpiece of carbon steel (0.30% carbon) used as cathode in place of K20 cemented carbides under the similar condition to that of Example 1. The thus coated workpiece was then dried in the air and heated at 1150 C. for 20 minutes in a hydrogen furnace. The resulting coating layer was lapped to thus give a beautiful golden color.

What is claimed is:

1. A method of making articles having a hard and attractive, golden colored-coating comprising dispersing a fine powder of tantalum carbide in a liquid medium selected from the group consisting of water, an aqueous medium and an organic solvent, immersing a workpiece of WC-base cemented carbides in the dispersion, applying a direct current voltage to the surface of said workpiece to cause electrophoretic deposition of the fine powder of tantalum carbide thereupon, removing the thuscoated workpiece from the dispersion and heating the coated surface at high temperatures in a vacuum or in a reducing or inert atmosphere, thereby forming a coating layer of tantalum carbide on the surface of the workp1ece.

2. The method of claim 1 wherein a small amount of at least one member selected from the group consisting of alginates, nitrates and sulfates is added to said dispersion.

3. The method of claim 1 wherein carbon powder is added to said fine powder.

4. The method of claim 1 wherein at least one transition metal is added to said fine powder as a binder metal.

5. The method of claim 1 wherein said workpiece is previously coated with at least one transition metal.

6. The articles having a hard and ornamental coating, produced by the method of claim 1.

7. The method of claim 1 wherein said workpiece con sists of tungsten carbide-cobalt.

8. The method of claim 1 wherein said workpiece consists of tungsten carbide-cobalt with at least one member selected from the group consisting of titanium carbide, tantalum carbide, molybdenum carbide, hafnium carbide, zirconium carbide, chromium carbide and vanadium carbide.

9. A method according to claim 1 wherein the coated References Cited UNITED STATES PATENTS 12/ 1948 Williams 25027.5

11/1951 Levin 204181 X 8/1958 Fahnoe et al 204181 X 10/1958 FahnOe et a1 204-181 X 10/1966 Lynch 11746 CC 2/1967 Lewis et a1 23-208 A X 10/1969 Ramirez 204181 X JOHN H. MACK, Primary Examiner A. C. PRESCOTT, Assistant Examiner US. Cl. X.R.

workpiece is heated to a temperature of at least 1000 C. 15 117-46 CB, 46 CC, 46 CG, 61, 93.4 

